LONDON - The passengers stepping off the Lufthansa flight from Frankfurt, Germany, last month head straight for the passport-scanning machines that allow European residents to enter Britain quickly and without any human interaction.

He has come here, as he does most months, to meet with WikiLeaks founder Julian Assange, the world's most controversial purveyor of government secrets. For most of the past six years, Assange has been confined to the Ecuadoran Embassy in London, fearful that if he leaves he will be extradited to the United States for prosecution under the Espionage Act. Ecuador recently granted Assange citizenship, but British officials said he is still subject to arrest if he leaves the embassy.

Müller-Maguhn is one of Assange's few connections to the outside world. He typically brings Assange books, clothes or movies. Once in 2016, he delivered a thumb drive that he says contained personal messages for the WikiLeaks founder, who for security reasons has stopped using email.

These visits have caught the attention of U.S. and European spy chiefs, who have struggled to understand how Assange's organization operates and how exactly WikiLeaks came to possess a trove of hacked Democratic Party emails that the group released at key moments in the 2016 presidential campaign.

The three major U.S. intelligence agencies - the CIA, the FBI and the National Security Agency - assessed "with high confidence" that Russia relayed to WikiLeaks material it had hacked from the Democratic National Committee and senior Democratic officials. And last year, then-FBI Director James B. Comey said that the bureau believes the transfer was made using a "cut-out," or a human intermediary or a series of intermediaries.

Exactly how the Russians delivered the email trove to WikiLeaks is the subject of an ongoing examination by U.S. and European intelligence officials. As part of their effort to understand the group's operations, these officials have taken an intense interest in Müller-Maguhn, who visits Assange monthly, U.S. officials said.

Müller-Maguhn insists that he was never in possession of the material before it was put online and that he did not transport it.

"That would be insane," he says.

U.S. officials who once dismissed WikiLeaks as a little more than an irritating propaganda machine and Assange as an antiestablishment carnival barker now take a far darker view of the group.

"It's time to call out WikiLeaks for what it really is: a nonstate hostile intelligence service," CIA Director Mike Pompeo said in the spring after the group released documents describing CIA hacking tools.In December, he doubled down on that assessment, describing WikiLeaks as a national security threat and suggesting that Assange cannot protect those who pass him state secrets.

"He ought to be a bit less confident about that," Pompeo said.

In an interview at the Ecuadoran Embassy last month, Assange insisted that Müller-Maguhn never possessed the hacked DNC emails and blasted Pompeo's statementsas "very strange and bombastic."

Müller-Maguhn is more cautious. "How many of you wouldn't be scared s---less by the head of the CIA declaring you the next target?" he asks.

The 46-year-old hacker moves through Heathrow Airport like a man who knows that powerful governments are tracking his every move. A Washington Post reporter travels with him as he goes through passport control.

He switches off his cellphone, fearful that British immigration officials have technology that can steal his data. Müller-Maguhn could enter the United Kingdom with his German identification card but prefers to use his passport. "The ID card has my address on it," he says.

A heavy-set immigration officer looks over Müller-Maguhn's passport and stares for several seconds at a computer screen.

"Why are you in the U.K?" he asks.

"I'm visiting people," Müller-Maguhn replies.

The officer pecks at his computer. Necks crane to catch a glimpse of the man clad in all black who is holding up the normally brisk line of passengers headed to early morning business meetings.

After a few minutes, the officer waves through Müller-Maguhn, who is walking toward the exit when the officer remembers one last question.

"Sir, sir, where are you traveling from again?" he shouts.

"Frankfurt," Müller-Maguhn replies.

And with that he is gone. Behind him, the immigration officer is still typing. The travelers who briefly took notice of Müller-Maguhn are back staring at their phones or marching toward their destinations. Müller-Maguhn heads for the Heathrow Express into London.

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The roots of Müller-Maguhn's relationship with Assange trace back to his teenage years in the 1980s when his walk to school in Hamburg took him past of the offices of the Chaos Computer Club.

The group embodied postwar Germany's anti-fascist convictions and the hacker underground's libertarian ethos. Now the largest hacker club in Europe, it bills itself as "a galactic community of life forms independent of age, sex, race or society orientation that strives across borders for freedom of information."

Müller-Maguhn soon became a friend, confidant and adviser to the group's founder, Wau Holland. "They were like a strange couple," said Peter Glaser, a club member, journalist and friend of both men. "Andy was very young and behaved like an adult, and Wau was older and behaved like a child."

Müller-Maguhn later parlayed his interest in computers and surveillance into a business he co-founded in 2003 making encrypted phones. He had hoped to sell the phones to journalists and dissidents but quickly discovered that military and intelligence agencies in Europe, Asia and the Middle East were the only clients who understood the technology and were willing to pay for it.

"This was during the time I was following the path of capitalism," he said with a smile during one of several lengthy interviews in Berlin.

Müller-Maguhn spent 10 years selling the phones before leaving the company. "You can imagine, I know really strange people in really strange places," he adds. These days, Müller-Maguhn says, he runs a data center that hosts websites and manages email for businesses. He also works as a security consultant, helping companies and governments safeguard their secrets. One of his clients is in China, a state known for its suppression of the internet and its surveillance of dissidents.

By Müller-Maguhn's calculus, the nominally communist government is less prone to violence overseas and less of a threat than the United States is. "They don't have the wish to apply their standards to the rest of the planet or have others dance to their music," he says. "So there's a big difference."

In recent years, Müller-Maguhn's consulting and advocacy work has carried him all over the world, including Moscow, where in 2016 and 2017 he attended a security conference organized by the Russian Defense Ministry.

On his way into London for his meeting with Assange, Müller-Maguhn casually mentions that he is just back from a three-day trip to Brazil.

"It was business-related," he says, declining to elaborate.

Müller-Maguhn hops out of a cab in Knightsbridge, a posh section of London that's home to Harrods department store, the Ecuadoran Embassy and Assange. On this cold December day, the stores are decked out for the Christmas season. Müller-Maguhn raises a camera with a telephoto lens and aims it at a building down the street from the brick embassy where Assange has been holed up since 2012.

The shutter on his Nikon camera clicks as he snaps a few shots, hoping to spot surveillance equipment pointed at Assange and the embassy. Women in fur coats rush by him as Bentleys and Rolls-Royces roll past on the busy road. Müller-Maguhn moves down the sidewalk to get a better angle, takes some more pictures and then slings the Nikon over his shoulder.

Farther down the block and closer to the embassy, he points up toward an apartment building where he suspects that the Spaniards, angry about Assange's tweets in support of Catalan separatists, may have set up a surveillance team.

Then he bounds up the steps of the building that houses the Ecuadoran Embassy, takes one last glance over his shoulder and rings the bell of the front door, where a guard immediately recognizes him and welcomes him inside.

Müller-Maguhn met Assange through the Chaos Computer Club in 2007 when the WikiLeaks founder was seeking support for his then-fledgling organization.

In those early days, Assange described his creation as a group committed to the mission of publishing original source material so citizens of the world could see "evidence of the truth" about global corporations and their governments.

Just past the doors to the embassy, a guard asks Müller-Maguhn to turn over all electronic devices: cameras, mobile phones, as well as his watch and car keys.

"The last time, they even looked into the fruit I was bringing," Müller-Maguhn says. "These guys have their job. They have their instructions. So I am not complaining."

Since WikiLeaks' early days, Assange's circle of contacts has contracted significantly. Some allies, such as Daniel Domscheit-Berg, who first invited Assange to the Chaos Computer Club and signed on as WikiLeaks' spokesman, broke with WikiLeaks in 2010 after Assange released hundreds of thousands of pages of U.S. military documents without redacting the names of local Afghans who had helped the military and could be targeted by the Taliban. Other backers were put off by Assange's legal troubles and allegations of sexual assault in Sweden or his Manichaean view of the world.

Still others alleged that the group allowed itself to be used as a tool by the Russians in their campaign to influence the 2016 U.S. presidential election.

"Look, he has messed up with so many people, I have no idea how many people he has left as friends," Müller-Maguhn says.

Assange continues to fear that he will be prosecuted by the United States and as a result is afraid to leave the embassy, saying that doing so would lead to his extradition. The Justice Department is considering a case against him, according to people familiar with the matter. Several months ago, Domscheit-Berg said, the FBI sought an interview with him in connection with a long-running grand jury investigation of WikiLeaks' publication of State Department cables. Domscheit-Berg said in an interview that he rebuffed the request. "No matter the differences that Julian and I had, I'm not going to talk to anybody about what happened," he said.

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As WikiLeaks has contracted and Assange has retreated from public view, it has become harder for Western intelligence agencies to get a sense of how the group operates. An internal CIA report from November said the U.S. intelligence community has "gained few good insights into WikiLeaks' inner workings." The agency predicted that Assange's negative views of Washington would lead the group to continue to "disproportionately" target the United States.

Former WikiLeaks supporters say the group is governed by Assange's whims. "The way to think of it is always just chaos," said one former WikiLeaks activist who spoke on the condition of anonymity to offer a frank opinion and avoid retribution from Assange. "There aren't any systems. There aren't any procedures - no formal roles, no working hours. It's all just Julian and whatever he feels like."

During the 2016 campaign, Assange put out word that he wanted material on Democratic nominee Hillary Clinton. "He was kind of asking everybody, 'Can we get something for the election?' " Müller-Maguhn recalls.

Assange signs off on all WikiLeaks publications but does not review everything that comes to the group. "For security reasons, he does not want that," Müller-Maguhn says. Müller-Maguhn, though, is vague about WikiLeaks' internal workings.

A former WikiLeaks associate said Müller-Maguhn and a colleague oversaw submissions through WikiLeaks' anonymous submission server in 2016 - although Müller-Maguhn denies such involvement.

The only reliable way to contact Assange, he says, is through Direct Message on Twitter. "He seems to live on Twitter," adds Müller-Maguhn, who doesn't hide his disdain for the platform. "On Twitter you follow people, and that's what German history forbids you to do," he says.

The size of WikiLeaks' staff and its finances are also murky. Neither Müller-Maguhn nor Assange will say how many people work for the group or where they are located. "It seems to be a rather small team," Müller-Maguhn says.

WikiLeaks has amassed a stash of bitcoin, a digital currency that enables anonymous, bank-free transactions. As of this week, the stockpile is worth about $18 million, although in late December, with the currency's spike in value, the group was sitting on $25 million, according to public online ledgers that record such transactions. Over the past several years, the Wau Holland Foundation, which was started in 2003 after the founder of the Chaos Computer Club died, collected hundreds of thousands of dollars for Assange's group.

Müller-Maguhn sits on the board of the foundation, which seeks to promote "freedom of information and civil courage in various forms." He says the foundation has provided support for some of WikiLeaks' releases, such as last year's "Vault 7" disclosure of CIA hacking tools.

He describes the Vault 7 releases as a public service, adding that the CIA was "messing up other people's computers and making it look like someone else had done it."

To Assange, any suggestion that Müller-Maguhn may have served as an intermediary to deliver the DNC emails is "a lame attempt" by U.S. intelligence agencies to hurt the Wau Holland Foundation, which is a key conduit for tax-free donations in Europe.

The threat is all the more significant because the only other source of tax-exempt donations, the U.S.-based Freedom of the Press Foundation, has cut ties to WikiLeaks.

Müller-Maguhn says he cannot say with certainty what was on the USB drive that he delivered to Assange. "How can I prove what was on there?" he says. "I cannot." But he adds that it would be risky and impractical to deliver sensitive files by hand, rather than through encrypted channels.

"A classical walk-in? You saw too many movies from the 1970s," he says.

These days, Müller-Maguhn describes his visits to the embassy as motivated by an increasingly rare commodity in Assange's world: friendship. Assange's visitors include celebrities, such as actress Pamela Anderson, and politicians, such as Nigel Farage, a vocal advocate for Britain's exit from the European Union, and Dana Rohrabacher, a GOP congressman from California.

When he talks to visitors, Assange turns on a white noise generator in the embassy conference room to counter listening devices. Above the door, he points out a surveillance camera and indicates that sensitive messages should be communicated only via handwritten notes, shielding the text from the camera with a hand or notepad cover.

"I had this s---ty impression of me standing there watching 50-year-olds making music for us, and Julian would still be there," he said.

After about two hours inside the embassy last month, Müller-Maguhn emerges from the building, carrying his black leather satchel, stuffed with documents, and his Nikon camera. He quickly makes his way through the Christmas crowds and back to Heathrow Airport for an evening flight home to Germany.

He tries to minimize his time in Britain. "I don't like to stay overnight in a country that is hostile toward me," he says.

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Jaffe reported from Washington. The Washington Post's Greg Miller, Rachel Weiner and Julie Tate in Washington, Karla Adam in London and Stefan Pauly in Berlin contributed to this report.